1. Field of the Invention
This invention relates in general to waveform analysis systems, and more particularly to a method and apparatus for compensating spectra and profiles derived therefrom for effects of drift.
2. Description of Related Art
The compositional analysis of thin films found in electronic devices is a challenging area for modern electronic miniaturization technology. As the dimensions of devices have become ever smaller, it has become increasingly more difficult to characterize them with sufficient detail to enable, not only good quality control in manufacturing, but also timely introduction to the marketplace through shortened product-development cycles. Spectroscopic analysis has been widely used to provide the critical compositional information required for accurate characterization of such devices. In particular, the application of electron spectroscopy to the analysis of thin films found in electronic devices is widely used for the characterization of these devices. In addition, these devices are frequently composed of alternating layers of insulating and conductive materials, such as oxides and metals, respectively, which can cause difficulties in the analysis.
Although analysis of conductors is usually not a problem for electron spectroscopy, insulators often charge and this causes shifts in electron spectra. Since the accurate location of peak position within a spectrum is crucial for identification of the constituents generating such peaks in order to obtain an accurate spectroscopic analysis, spectral shifts are highly undesirable and have deleterious effects on quantification, especially compositional profiles, when it is based on spectra exhibiting such shifts. Such shifts become most troublesome, when linear-least-squares (LLS) or factor analysis (FA) methods are required to profile differing chemical species falling in the same spectral range.
When structures consisting of layered conductors and insulators are analyzed using physical delayering techniques, such as sputter-depth profiling analysis, the charging problem can be so severe as to preclude analysis. This is particularly true of sputter-depth profiling analysis with Auger electron spectroscopy. Compositional depth profiles display the relative contribution from various constituents with depth below the initial surface of a sample based on a sequential series of spectra obtained from strata exposed by removal of overlayers through sputtering of the sample with a rastered ion beam. In the case of Auger depth profiling, embedded insulating layers often charge negatively so that conventional techniques of charge neutralization by electron flood guns are unusable, which results in corrupted depth profiles of the distribution of constituents with depth below the surface of the sample.
A dramatic example of this problem arises in the analysis of certain oxide structures in metal oxide semiconductor field effect transistor (MOSFET) devices, which employ layered structures of insulating SiO2 on a semiconducting silicon substrate. MOSFET devices are the transistorized switches that comprise the principal circuit components of integrated circuits employed in modern digital computers. Depth profiles through such structures can demonstrate severe shifts in the spectra from strata in the SiO2 layer exposed during sputter-depth profiling analysis; and these shifts confound accurate analysis of such structures, their compositional depth profiles and especially compositional depth profiles requiring the use of LLS or FA analysis procedures.
Problems similar to those that arise in Auger electron spectroscopy can also arise with other electron spectroscopic techniques, such as x-ray photoelectron spectroscopy, and electron energy loss spectroscopy. Not only can charging of the sample cause spectral shifts, but also a variety of other things can cause spectral shifts, ranging from mechanical instabilities of the sample to thermal fluctuations within the spectrometer. More generally, the term spectral drift refers to spectral shifts due to these various sources. It would be useful to have a universally applicable method for removing the effects of such varied sources of drift from influencing the analyses based on sequential series of such drifted spectra, particularly compositional depth profiles. In some cases, for example, in the analysis of unique one-of a-kind samples, as often arises in the failure analysis of localized defects within a microelectronic circuit, it is absolutely essential to have some means for compensating for the effects of drift even when all that are available are inferior spectra that have been corrupted by the effects of drift. In such cases, the defect is often sacrificed, i.e. destroyed, in the course of a sputter-depth-profile analysis, leaving no second chance to obtain drift-free spectra. As a result, critical manufacturing problems that might potentially result in significant yield losses and increased costs of manufacturing can go unresolved, unless such drifted spectra can somehow be compensated for the effects of drift to provide a useful analysis.
It can easily be seen that, just as spectral drift can influence the quality of spectra obtained by other electron spectroscopic techniques, spectral drift can also deleteriously influence spectra acquired with other non-electron spectroscopic techniques. More generally, spectral drift is a bad thing for any form of spectroscopy. Even more generally, a spectrum can be viewed as an electronic signal or waveform, and spectral drift, as a phase shift of the signal or waveform that produces a lead or a lag along the time axis. Hence, a universally applicable technique for removing the effects of such drift from a sequential series of spectra obtained in spectroscopy has benefits in a wide variety of other applications, especially in signal processing and waveform analysis.
It can be seen then that there is a need for a method for compensating a sequence of spectra, or waveforms, for the effects of spectral drift or phase shift, respectively.
It can also be seen that there is a need for a method compensating compositional profiles for the effects of drift because compositional profiles are derived from sequences of such spectra.
It can also be seen that there is a need for an apparatus for compensating a sequence of spectra, or waveforms, and for compensating profiles derived therefrom for the effects of drift, specifically, a spectroscopic analysis system, or more generally, a waveform analysis system.